1. Field of the Invention
This invention relates to a silicon thin film transistor which is mainly used in an active-matrix driving type of liquid crystal display, an image sensor, a thermal head, etc., and a method for forming the same.
2. Description of Related Art
A silicon thin film transistor (hereinafter referred to as "TFT") has been conventionally used in a liquid crystal display for a compact liquid crystal television and a computer, or an image sensor and a thermal head which are used in a facsimile, etc. A method for producing a conventional coplanar type of crystal silicon TFT will be hereunder described. First, an amorphous silicon layer is formed on an insulating substrate such as a glass substrate or the like, and then subjected to a heat annealing treatment at 550.degree. C. to 800.degree. C. or a laser irradiation to crystallize the amoriohous silicon layer. Subsequently, the crystallized silicon layer is subjected to a photolithographic process to be formed in an islandish shape. The islandish silicon layer serves as a channel forming region and source/drain region. In addition, a silicon oxide layer serving as a gate oxide layer is further formed on the islandish silicon layer by a sputtering method, a CVD method or the like. Subsequently, a gate electrode layer is formed, then a patterning process is conducted, then an impurity doping is conducted by an ion implantation method to make the source/drain regions one conductive type, and then the impurity is activated. Finally, a hydrogen heat treatment, formation of interlayer insulating films, formation of contact holes, and then formation of aluminum electrodes are performed to complete the coplanar type TFT.
The conventional coplanar type TFT thus formed has the structure as shown in FIG. 2A-1 to 2A-3. In FIG. 2A-1, a cross-sectional view of the structure is shown in FIG. 2A-1 which is taken along a line 2A-3--2A-3 shown in FIG. 2A-3, and a cross-sectional view or the structure which is taken along a line 2A-2--2A-2 is shown in FIG. 2A-2.
In FIG. 2A-2-2A-3, a reference numeral 20 represents a glass substrate, a reference numeral 21 represents a crystal silicon layer constituting the channel forming region and the source/drain region, a reference numeral 22 represents a silicon oxide layer serving as a gate insulating film, a reference numeral 23 represents a gate electrode, a reference numeral represents aluminum wiring for a source/drain electrode, and a reference numeral 25 represents an interlayer insulating film. As not shown, a silicon oxide film is coated as a protection film on the glass substrate to prevent diffusion of the impurities from the glass substrate.
In a TFT using crystal silicon, the crystal silicon layer which serves as an active layer constituting the channel forming region and the source/drain region is formed by crystallizing an amorphous silicon layer through heating. It has been known that the crystal silicon has more excellent crystallinity and more excellent electrical characteristics as the thickness of the amorphous silicon layer serving as starting material in thicker. However, if the thickness of the silicon semiconductor layer (hereinafter referred to as "active layer") constituting the channel forming region and the source/drain region which is shaped in a islandish form as described above is increased, a large step portion is formed at the edge portion of the islandish active layer Therefore, when the silicon oxide film layer serving as the gate insulating film is formed on the silicon active layer, the thickness of the silicon oxide layer at the side surface of the step portion becomes thinner. This is schematically shown in FIG. 2(B). FIG. 2(B) corresponds to the cross-sectional view of FIG. 2A-1 which is taken along the line. In FIG. 2(B), the interlayer insulating film 25 of FIG. 2A-2 is not shown.
Generally, the sputtering method has been frequently used to form the silicon oxide film layer. However, when the sputtering method is used, in the islandish active layer 21 having a steeply rising-up edge portion as shown in FIG. 2 (B), particularly, the thickness of the insulating film layer at the side surface thereof becomes thinner as indicated by a dotted circle 26 of FIG. 2 (B) because the step coverage of the insulating film layer at the side surface of the islandish active layer 21 is insufficient. Particularly when the thickness of the active layer 21 is increased, the insulating film layer at the side surface of the edge portion of the active layer is remarkably thin. Therefore, the conventional TFT has disadvantages that the active layer 21 and the gate electrode 23 are liable to be short-circuited because the insulating film layer is cut off at the step portion thereof, and that even if the insulating film layer is not cut off at the step portion, an electric field is concentrated on the insulating film layer at the corners of the edge portion of the active layer, so that the voltage resistance (dielectric strength) between the gate electrode and the source/drain electrode is reduced.
These disadvantages have mainly caused the lowering of yield and degradation in quality particularly for an active type of liquid crystal display device, an image sensor, etc. which require a number of TFTs to be provided on the glass substrate.
As one method of solving the above problem, there is proposed a method that the thickness of the silicon oxide film at the edge portion of the active layer is practically sufficiently increased by thickening the silicon oxide film layer serving as the gate oxide film. However, the thickening of the gate oxide film affects the electrical characteristics of the TFT, and thus the thickness of the gate oxide film can not be increased limitlessly. On the other hand, there is another method that the gate insulating film is formed by a film forming method providing excellent step coverage to solve the above problem in thickness of the insulating film layer at the side surface of the edge portion of the active layer. As this type of method have been known a photo-CVD method, a heat-CVD method and so on, however, the photo-CVD has a problem in producibility and the heat-CVD method has a problem that a low-cost glass substrate can not be used because this method requires a temperature condition above 650.degree. C.